GeV gamma-rays and TeV neutrinos from very massive compact binary systems: the case of WR 20a

Abstract

Massive Wolf–Rayet stars in a compact binary system are characterized by very strong winds which collide, creating a shock wave. If the wind nuclei, from helium up to oxygen, accelerated at the shock can reach large enough energies, they suffer disintegration in collisions with soft thermal radiation from the massive stars injecting relativistic protons and neutrons. Protons collide with the matter of the wind and a fraction of neutrons collide with the massive stars producing γ‐rays and neutrinos via decay of pions. We calculate γ‐ray fluxes from inverse Compton e± pair cascades initiated by primary γ‐rays and leptons, which are produced by protons, and the neutrino fluxes, produced by protons and neutrons, for the example compact massive binary WR 20a. From normalization of these cascade γ‐ray spectra to the fluxes of the EGRET sources 2EG J1021 − 5835 and 2EG J1049 − 5847 observed in the direction of WR 20a, we conclude that this massive binary can be detected by the IceCube type of neutrino detector. The estimated muon neutrino event rate inside the 1‐km2 detector during 1 year is between a few and a few tens, depending on the spectrum of accelerated nuclei.

The Fermi-LAT view of the colliding wind binaries

“…The acceleration takes place in a region where the energy density of the photon and magnetic fields is very high. This leads to a production of a strong non-thermal radiation via synchrotron and inverse Compton mechanisms (Eichler & Usov 1993;Dougherty & Williams 2000;Benaglia & Romero 2003;Bednarek 2005;Reimer et al 2006;Pittard & Dougherty 2006;Reimer & Reimer 2009;Reitberger et al 2014). Another contribution to the high energy (HE) part of the spectrum could arise due to the presence of accelerated hadrons that could interact with wind material, producing pions.…”

ηCarinae: a very large hadron collider

“…Proton-proton interactions and subsequent pion decay. Bednarek (2005) pointed out that high-energy hadrons in the wind of massive stars will be photo-disintegrated into A57, page 4 of 7 protons and neutrons on a short timescale. The interaction timescale for protons is inversely proportional to the density of matter in the post-shock region, where the particles are trapped by the magnetic field.…”

“…Hydrodynamic simulations indicate that the material flows out of the shock region at a velocity smaller than, but of the order of, the pre-shock wind velocity (Pittard 2009). As the shock region has a size similar to the stellar separation, the bulk diffusion timescale can be estimated as (Bednarek 2005) t bulk = 3R/V ≈ R 10 14 cm V 10 3 km s −1 × 3 × 10 6 s. with t IC and t pp for electrons and protons respectively (where R L is the Larmor radius) provides the maximum characteristic energy of the particle distribution.…”

Revisiting the Westerlund 2 field with the HESS telescope array

“…Different scenarios have been proposed to account for VHE γ-ray emission from stellar environments, considering the contribution of hot and massive Wolf-Rayet (WR) and OB stars and their winds in single, binary, or collective processes, pulsars and their synchrotron nebulae, as well as supernova explosions and their expanding remnants (Eichler & Usov 1993;White & Chen 1995;Benaglia & Romero 2003;Reimer et al 2006;Pittard & Dougherty 2006;Torres et al 2004;Bednarek 2005;Manolakou et al 2007). The giant H ii region RCW 49 (NGC 3247) and its ionizing cluster Westerlund 2, hosting over two dozen of massive stars as well as two remarkable WR stars (WR 20a and WR 20b), have been studied extensively over the whole electromagnetic spectrum, and certainly interest was further boosted by the discovery of VHE γ-ray emission from the vicinity of Westerlund 2.…”